Delta-sigma digital radiometric system

BACKGROUND

Field

This invention relates generally to a radiometric system for passively detecting and thermally imaging objects in a scene and, more particularly, to a radiometric system for passively detecting and thermally imaging objects in a scene, where the system employs a digital square-law quantizer and delta-sigma feedback integrated on a common chip using, for example, silicon-germanium (SiGe) and/or Si complementary metal oxide semiconductor (CMOS) fabrication technologies.

Discussion

Radiometric thermal imaging systems and cameras are known in the art that passively detect and process signals in certain frequency bands emitted from a particular scene. Some of these frequency bands, such as various RF bands in the 60-300 GHz frequency range, for example, 94 GHz (W-band), 140 GHz (D-band) and 220 GHz, are particularly useful because the emissions in those bands readily propagate through clouds, smoke, dust, fog, etc. Typically, the warmer or more emissive the object in the scene, the more energy the object emits at a particular frequency. The radiometric system will detect the RF energy and convert it to a representative temperature value so that the different objects in the scene can be separately imaged and identified. In order to be suitable and effective, these radiometric systems generally require detectors that have a high sensitivity, low drift and can handle large background temperatures.

A typical radiometric camera that detects and images radiation in the RF bands often includes a focal plane array (FPA) that converts the radiation into an electric signal, where a lens focuses the radiation onto the array. The FPA typically includes a configuration of a plurality of receivers positioned in a two-dimensional plane, where each of the receivers includes an antenna or signal horn having a pick-up probe at the front end that converts the radiation to an electrical signal that is amplified by a microwave monolithic integrated circuit (MMIC) low noise amplifier. A diode at the back end of the each receiver rectifies the amplified voltage signal to a DC signal, where the DC signal amplitude is representative of the power level of the received signal, which increases as the temperature of the object being imaged increases, and where power and temperature are proportional to each other. The DC voltage signal from each receiver is then digitized and converted to an image, where higher voltages are displayed as whiter areas in the image representing warmer objects with higher radiometric temperature.

One of the challenges associated with these types of radiometric cameras is providing a high enough thermal resolution between objects detected in the scene. For example, in a typical scene, many of the objects will be near room temperature, thus requiring significant temperature resolution to distinguish those objects from each other. The sensitivity of the radiometer system depends on the receive system noise temperature T, the captured radiometric bandwidth B and the integration time τ as follows where it is assumed that the detector does not contribute input-referred noise power, and that low frequency bias drift is not present.

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It is desirable that the operation of the radiometric system be as immune from signal drift as possible as the temperature of the system itself changes. In order to overcome a significant portion of this analog drift, it is known in the art to provide digital radiometric systems where the received RF analog signal is converted to the digital domain as quickly as possible. However, there are advantages and disadvantages to providing different levels and degrees of analog circuitry and digital circuitry in these types of systems. For example, the antenna and amplifier capture signals over a wide range of frequencies B. Analog diodes can rectify an entire broad frequency range. However, these types of analog circuits that employ diodes and the like often times have unacceptable signal drift and noise, especially at lower frequencies. Further, diode detectors typically do not have enough nonlinearity and are inefficient. Also, known digital radiometric systems that may employ fast Fourier transform (FFT) or root mean square (RMS) operations that convert digital frequency signals to a representative temperature typically require significant calculation at high sampling and data rates.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram of a digital radiometric receiver architecture employing digital square-law detection and delta-sigma feedback; and

<figref idref="DRAWINGS">FIG. 2</figref> is a flowchart diagram of a method for thermally imaging objects in a scene using digital square-law detection and delta-sigma feedback.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed to a passive digital radiometric thermal imaging system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, although the discussion herein talks about the radiometric system detecting RF and millimeter wave signals, the invention has application for a wider signal range, such as into the terahertz (THz) frequency bands.

The present invention proposes a digital radiometric system for thermally imaging a scene by passively detecting RF energy emitted from the scene and converting that RF energy to a thermal image. Through advances in silicon-germanium (SiGe) and Si complementary metal oxide semiconductor (CMOS) fabrication technologies, effective digital circuits can be built and integrated on a single chip so that the RF signals can be effectively converted to representative thermal signals with high resolution and for higher frequency ranges. As will be discussed, the radiometric system of the invention employs square-law detection to convert the RF signals to a representative digital signal, which employs a feedback signal from a delta-sigma circuit, where the square-law detector and the delta-sigma circuit are fabricated on a common chip using SiGe and Si CMOS fabrication technologies. The digital front-end of the square-law detector minimizes drift in the receive signal as a result of temperature changes of the system, and provides the sensitivity required for high resolution. In one embodiment, the square-law detector provides high sensitivity, such as around 1 Kelvin, has low drift, such as under 100 PPM, and can handle large background and amplifier noise temperatures, equivalent to as much as 3000 Kelvin.

<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram of a receiver architecture <b>10</b> for a passive digital radiometric system. The architecture <b>10</b> includes an antenna <b>12</b> that accepts the RF radiation from a scene at the desired frequency band of interest, where the antenna <b>12</b> is configured and designed for that particular frequency band. Signals received by the antenna <b>12</b> are provided to a receiver front-end <b>18</b> where they are amplified by a low noise amplifier (LNA) <b>14</b> and then down-converted to an intermediate frequency by a down-conversion circuit <b>16</b> to convert the high frequency received by the antenna <b>12</b> to a lower frequency more suitable for operation by the digital circuitry of the architecture <b>10</b>, as will be discussed below. For example, the antenna <b>12</b> may be designed to detect frequencies in the 90 GHz frequency band, and the circuit <b>16</b> may convert that frequency down to about 10 GHz, where the lower frequency still provides high resolution and a low enough frequency for operation of the digital circuitry. Employing the down-conversion circuit <b>16</b> would typically be design specific in that requiring such circuitry would depend on the frequency band being detected and the speed of the available digital electronics.

It is noted that the receiver architecture <b>10</b> and the antenna <b>12</b> may be one receiver architecture and antenna for one pixel of an array of many receiver architectures, where the antenna <b>12</b> may be part of a focal plane array (FPA). A typical FPA may include a configuration of a plurality of receivers positioned in a two-dimensional plane, where each of the receivers includes an antenna or signal horn having a pick-up probe. Alternately, the antenna <b>12</b> may be scanned across a scene through some scanning procedure, where the system only includes a single receiver architecture. It is also noted that for simplicity a unipolar circuit architecture is shown, which processes only radio signals above zero volts. One skilled in the art would have no difficulty constructing a bipolar equivalent handling both positive and negative threshold levels.

The down-converted RF signal from the receiver front-end <b>18</b> is sent to a square-law quantizer circuit <b>20</b> that converts the analog signal to a representative digital signal. The square-law quantizer circuit <b>20</b> includes a plurality of digital bit channels <b>22</b> each including a comparator <b>24</b> having a positive input and a negative input, where the RF signal from the receiver front-end <b>18</b> is provided to the positive input of each of the comparators <b>24</b>, and where the output of each comparator <b>24</b> represents a digital bit that is part of a digital word identifying the power of the incoming RF signal at any particular sample point in time. More particularly, if the power of the RF signal at the positive input of the comparator <b>24</b> is higher than a reference voltage at the negative input of the comparator <b>24</b>, then the output of the comparator <b>24</b> goes high representing a digital one bit, where otherwise the output of the comparator <b>24</b> is low representing a digital zero bit.

The square-law quantizer circuit <b>20</b> also includes a voltage divider network <b>26</b> having a resistor ladder <b>28</b>. Each of the resistors <b>28</b> is weighted with a different resistive value, where in this non-limiting example, the lower resistor has the highest value and where the resistive value of the resistors <b>28</b> decreases towards the top of the resistor ladder <b>28</b>. For example, in one non-limiting embodiment, the lowest of the resistors <b>28</b> may have an ohmic value of R, the second lowest of the resistors may have an ohmic value of 0.414 R, the next resistor <b>28</b> may have an ohmic value of 0.318 R, and the top resistor <b>28</b> may have an ohmic value of 0.268 R. A current flow provided on a feedback line <b>30</b> discussed in detail below flowing through the voltage divider network <b>26</b> provides a different voltage drop across each of the resistors. The negative input of each of the comparators <b>24</b> is coupled to a different location in the voltage divider network <b>26</b> so that a different potential is provided to the negative input of each of the comparators <b>24</b>.

In this design, the negative input of the comparators <b>24</b> is coupled to the voltage divider network <b>26</b> so that the lower comparator <b>24</b> provides a logic one bit output for the lowest energy threshold level of the RF signal, the next comparator <b>24</b> provides a logic one bit output for a next increase in power level of the RF signal, the next comparator <b>24</b> provides a logic one bit output for a next increase in the power level of the RF signal and for the higher power RF signals, all of the comparators <b>24</b> will provide a logic one bit output. Although this embodiment includes four of the bit channels <b>22</b>, this is by way of a non-limiting example in that the number of the channels <b>22</b> would be application specific. For example, an increase in the number of channels <b>22</b> may increase the bit resolution for a particular system, but increases the electronics required to process the signals, and at some point there is a diminishing level of return for noise reduction and resolution.

The digital bits from the square-law quantizer circuit <b>20</b> are sent to a weighting and offset table <b>40</b> that performs mathematical operations on the digital signal that then outputs the modified digital signal as a digital data stream on line <b>42</b>. The weighting table <b>40</b> normalizes the digital word based on the outputs from the comparators <b>24</b> and the values of the resistors <b>22</b> so that the digital signal is a value that falls within a particular range, such as zero to 1. The digital data stream on the line <b>42</b> is sent to an accumulate and decimate circuit <b>44</b> that accumulates the weighted data bits over time and then divides the accumulated bits by the number of samples to provide an averaged digital signal representative of the temperature at any particular point in time, where the circuit <b>44</b> operates as a long term averaging filter. More particularly, the circuit <b>44</b> operates to filter out noise in the received RF signal at a particular point in time by averaging the temperature signal over time to remove the noise. By averaging many of the digital signals over time, the resolution of the detected power signal from the antenna <b>12</b> can be increased. In this way, for example, the square-law quantizer <b>20</b> processes billions of samples per second and the circuit <b>44</b> outputs thousands of samples per second to provide the relatively noise free resolution. The normalized digital signal from the circuit <b>44</b> provided on line <b>46</b> is the actual digital signal that is then converted to the thermal image, where that signal is provided on line <b>50</b> for subsequent image processing in a manner that would be well understood by those skilled in the art.

The weighting table <b>40</b> and the circuit <b>44</b> combine to operate as a delta-sigma circuit to provide a feedback signal on the line <b>30</b> to the voltage divider network <b>26</b> so that the voltage potential across the resistors <b>28</b> changes depending on the power of the signal being received at any particular point in time. In order to provide the feedback signal on the line <b>30</b>, each digital signal that is output from the circuit <b>44</b> on the line <b>46</b> is converted to an analog signal by a digital-to-analog converter (DAC) <b>48</b>. If the particular RF signal provided by the front-end <b>18</b> is at a higher end of the power range, then the feedback signal on the line <b>30</b> will change the voltage values applied to the negative input of the comparators <b>24</b> so that the output of the comparators <b>24</b> will not go high until a higher RF value is received, thus increasing the resolution of the system at the particular power level being received at any point in time. Thus, by changing the voltage values provided to the comparators <b>24</b> through the feedback line <b>30</b>, the balance and ratio between the different comparators <b>24</b> is maintained over a wider input power range.

<figref idref="DRAWINGS">FIG. 2</figref> is a flowchart diagram <b>100</b> of a method for thermally imaging objects in a scene using digital square-law detection and delta-sigma feedback. At box <b>102</b>, the antenna <b>12</b> accepts RF radiation signals from a scene and provides a power signal to the front-end <b>18</b>. At box <b>104</b>, the power signal is converted to a digital signal in the square-law quantizer circuit <b>20</b> described above. At box <b>106</b>, the digital signal from the comparators <b>24</b> in the quantizer circuit <b>20</b> is averaged over time in a delta-sigma circuit including the weighting table <b>40</b> and the accumulate and decimate circuit <b>44</b>. The output of the circuit <b>44</b> is the temperature representation of the scene. At box <b>108</b>, the averaged digital signal output from the circuit <b>44</b> is converted to an analog feedback signal in the DAC <b>48</b> and the feedback signal is provided to the quantizer circuit <b>20</b> as discussed earlier.

The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.